The aggressive and unpredictable nature of relapsed T-cell acute lymphoblastic leukemia (T-ALL) represents a major clinical challenge, in large part due to highly toxic and ineffective chemotherapies. Further advances in therapy will require a detailed understanding of the molecular underpinnings of T- ALL development and relapse. To this end, I have completed a highly innovative screen in zebrafish to identify genes associated with T-ALL progression and have found that the protein tyrosine phosphotase PRL-3 was commonly amplified in single T-ALL cells as they developed increased ability to form relapse over time. In transgenic animals, PRL-3 over-expression significantly enhanced primary T-ALL development and relapse formation. I have found that PRL-3 amplified in a subset of human T-ALL and is highly expressed by a majority of T-ALL patient samples. These cells are also sensitive to PRL-3 knock-down through the induction of apoptosis. These results imply that PRL-3 activity has an important role in T-ALL malignancy, and that PRL-3 and its substrates may represent novel therapeutic targets for the treatment of T-ALL. The objectives of this proposal are to 1) define the contribution of PRL-3 to T- ALL progression and relapse and 2) identify the mechanism by which PRL-3 drives T-ALL malignancy. This objective will be achieved through two specific aims. During the K99 phase of this award, I will complete Aim 1, where I will examine the effect of PRL-3 gain-of-function and loss-of-function in various stages of T-ALL development in zebrafish, including invasion from the thymus, intravasation, proliferation and relapse, and I will also test the effects of PRL-3 knock-down on human cells in a murine xenograft model. During this phase, I will learn techniques to directly image cancer progression in live animals and to develop orthotopic leukemia xenografts in mice. With this knowledge in hand, during the R00 phase I will complete Aim 2, where I will identify the target substrates of PRL-3 in T-ALL cells, and test the contribution of these genes and pathways to T-ALL malignancy using in vivo and in vitro epistasis experiments. In total, this work will lay the foundation for the development of small molecule PRL-3 and pathway inhibitors for potential therapeutic use in T-ALL. The mentored phase of this award will occur under the guidance of Dr. David Langenau, an expert in zebrafish models of cancer, and Dr. Thomas Look, an internationally recognized leader in the field of leukemia research. Their proven track record of mentorship, coupled with an intensive didactic component and the rigorous and nurturing academic environment offered by the research community of Massachusetts General Hospital and Harvard Medical School offer the best opportunity for my success as I transition to becoming an independent investigator.